1. Field of the Invention
The present invention generally relates to a receiving apparatus and a receiving method in which a receiving operation is intermittently performed. More particularly, the invention relates to a receiving apparatus suitably used in a mobile communication system which performs communications according to, for example, a code division multiple access method. The invention is also concerned with a receiving method for use in the above type of receiving apparatus.
2. Description of the Related Art
Hitherto, in performing communications by sharing a single base station among a plurality of mobile stations, a communication method, such as frequency division multiple access, time division multiple access, or code division multiple access, is employed in order to avoid interference between the communication lines of the individual mobile stations. Each of the above methods has advantages and disadvantages and is thus selected according to the intended purpose of the use of the communication system.
For example, in the code division multiple access (hereinafter referred to as xe2x80x9cCDMAxe2x80x9d) method, a modulated wave of the same carrier frequency is spread into a wider band than the original band by utilizing a specific code (hereinafter referred to as xe2x80x9cPseudo-random Noise sequence (PN) codexe2x80x9d) assigned to each line. The above frequency spread is referred to as the xe2x80x9cspread spectrum techniquexe2x80x9d. Then, the respective modulated waves processed by the spread spectrum technique are multiplexed and then transmitted. On the receiving side, the received spread spectrum signal is synchronized with the same PN code as that of the transmitting side which is supplied via a predetermined line. Thus, only a desired line can be identified.
In the spread spectrum technique, the same PN code as that of the transmitting side with respect to the phase of a receiving signal as well as its pattern, is required for despreading the signal. Thus, multiple access is made possible by changing the pattern or the phase of the PN code. In this manner, multiple access is implemented by varying the pattern or the phase of the code sequence of the spread spectrum signal. This is referred to as the CDMA method.
In the CDMA method, different PN signals are first assigned to the respective lines. The transmitting side multiplies the respective modulated waves to be transmitted via the individual lines by the different PN codes, thereby performing spread spectrum. It should be noted that the individual waves have been modulated in a predetermined manner prior to spread spectrum processing. The modulated waves processed by the spread spectrum technique are then multiplexed and transmitted.
The receiving side multiplies the receiving signal transmitted from the transmitting side by the same PN code as that assigned to the predetermined line while synchronizing the signal with the PN code. This makes it possible to demodulate the modulated wave transmitted only through a desired line.
In this manner, according to the CDMA method, communications can be directly established between the transmitting side and the receiving side if the same code is set on both sides. Moreover, in the CDMA method, spread spectrum is performed, as discussed above, on the modulated waves by using the different PN codes assigned to the individual lines. This allows the receiving side to demodulate the spread spectrum signal transmitted only via a predetermined line, and additionally, PN codes are pseudo random sequence codes, thereby ensuring communication privacy.
Further, in the mobile communication system using the CDMA method, the base station on the transmitting side repeatedly transmits a PN code for allowing a mobile station to establish and maintain synchronization and to reproduce the clock. The PN code, which is repeatedly transmitted from the transmitting side, is referred to as xe2x80x9ca pilot signalxe2x80x9d. The mobile station on the receiving side then detects the respective pilot signals transmitted from a plurality of base stations and allocates the detected timings to the respective demodulators. It should be noted that a PN code is generated within the demodulators of the mobile station. Each demodulator then multiplies at an allocated timing the spread spectrum signal transmitted from a desired base station by the PN code, thereby demodulating the receiving signal.
Namely, in the mobile communication system using the CDMA method, the respective base stations transmit PN codes having different timings as pilot signals. In response to the transmitted pilot signals, the mobile station detects the timing of the pilot signal supplied from a desired base station and synchronizes the PN code generated in the demodulator of the mobile station with the detected timing. As a consequence, the spread spectrum signal transmitted only from a desired base station can be correctly demodulated.
Although the individual base stations respectively transmit the PN codes having different timings, as discussed above, all the PN codes have the same code pattern. A timing difference between the pilot signals transmitted from the respective base stations appears to be a difference between the PN codes.
In the mobile station, while being in the standby position, an intermittent receiving operation may be performed to save power. In the mobile communication system using the CDMA method, the intermittent receiving operation is performed in the following manner. Prior to the start of a receiving zone, pilot signals are obtained to re-establish synchronization with the respective base stations. The detected timings are then assigned to the respective demodulators. Upon completing the receiving zone, the mobile station interrupts the receiving operation, thus losing synchronization with the base stations. Thus, signals can be intermittently received by the mobile station by repeating the above-described operation. With a longer period of the intermittent receiving zone (in other words, if a non-receiving zone is longer), more effective power conservation is expected.
However, the above-described intermittent receiving operation presents the following problems. In the CDMA mobile station, with a longer period of the intermittent receiving zone (if the non-receiving zone is longer), errors made by the oscillator within the mobile station are accumulated, thereby increasing the possibility of failing to establish synchronization with the pilot signal (i.e., failing to detect the timing) prior to the intermittent receiving operation. Namely, the intermittent receiving operation cannot be accurately performed.
The aforementioned problem may be solved by using a high-precision oscillator. This, however, increases the cost of the mobile station, which serves as a receiving apparatus. On the other hand, without using a high-precision oscillator, it is very difficult for the mobile station to perform the intermittent receiving operation having a longer period. It is thus hard to obtain a low-powered receiving apparatus.
Accordingly, in order to solve the foregoing problems, it is an object of the present invention to provide a low-powered receiving apparatus in which an intermittent receiving operation having a longer period can be performed in a mobile station even by the use of a low-precision and simple-structured oscillator, and also to provide a receiving method for use in the above type of receiving apparatus.
To achieve the above object, according to one aspect of the present invention, there is provided a receiving apparatus for use in a mobile communication system which transmits and receives spread spectrum signals between a base station and a mobile station according to a code division multiple access technique. The receiving apparatus includes timing detection means for detecting, in re-synchronizing with the mobile communication system, a timing of a pilot signal transmitted from the base station by using the pilot signal and a spread spectrum signal generated corresponding to the pilot signal. Intermittent receiving means performs an intermittent receiving operation based on the timing detected by the timing detection means. Control means sets, in accordance with an intermittent receiving period set in the intermittent receiving means, a region of the spread spectrum signal within which the timing is detected by the timing detection means.
According to another aspect of the present invention, there is provided a receiving method for use in the above type of receiving apparatus. The receiving method includes the steps of detecting, in re-synchronizing with the mobile communication system, a timing of a pilot signal transmitted from the base station by using the pilot signal and a spread spectrum signal generated corresponding to the pilot signal, and of performing an intermittent receiving operation based on the timing detected in the detection step, the timing being detected by setting a timing detection region in accordance with an intermittent receiving period of the intermittent receiving operation.
Errors of the oscillator are accumulated over a long intermittent receiving period, which may fail to establish synchronization. Thus, the precision of the timing detecting operation, for example, the detection capability used as the search area, is set according to the intermittent receiving period, thereby compensating for errors of the oscillator of the mobile station. As a result, a low-powered intermittent receiving operation can be performed even over a long intermittent receiving period.